This invention relates to a process for preparing 4,6-diamino-1,3-benzenediol (4,6-diaminoresorcinol). More specifically, it relates to the synthesis of 4,6-diaminoresorcinol starting with 1,3-benzenediol (resorcinol).
Diaminoresorcinols are useful as monomers for the preparation of polybenzoxazoles (PBOs), the utility of which is discussed by, for example, Wolfe in Mark et al., "The Encyclopedia of Polymer Science and Engineering", vol. 11, pp. 601-635, Wiley-InterScience Publication, New York, 1988. One of the more efficient and economical methods of preparing 4,6-diaminoresorcinol had been Lysenko's process (U.S. Pat. No. 4,766,244, herein incorporated by reference) which reported the synthesis of the desired product in high purities and yields in three steps from 1,2,3-trichlorobenzene.
Unfortunately, halogenated organic compounds--including halogenated arenes--have become the subject of close environmental scrutiny in recent years. Consequently, the once inexpensive and plentiful 1,2,3-trichlorobenzene is becoming expensive and difficult to obtain. The questionable long-term availability of 1,2,3-trichlorobenzene requires that an inexpensive commodity starting material be found to prepare 4,6-diaminoresorcinol.
Lysenko et al. addresses this problem somewhat in U.S. Pat. No. 4,982,001, herein incorporated by reference, by preparing 4,6-diaminoresorcinol from the inexpensive and readily available resorcinol by way of a 1,3-bis(methylcarbonato)benzene intermediate. The steric hindrance of this intermediate causes nitration to take place mostly at the 4- and 6-positions, so that the desired 4,6-diaminoresorcinol can be prepared upon hydrolysis and hydrogenation. Unfortunately, a significant degree of undesirable nitration occurs at the 2-position of the intermediate, making this process impractical.
In view of the deficiencies in the art, it would be desirable to prepare 4,6-diaminoresorcinol from resorcinol by a process where undesirable nitration does not occur.